Preserve existing dylib paths before calling spirv-builder

[bspeice]

In #20, spirv-builder-cli sets up the environment to load rustc_codegen_spirv, but does so without preserving existing paths. Because spirv-builder-cli sets the PATH variable on Windows, spirv-builder is then unable to find the cargo command, causing the issues seen in #50.

Instead, prepend the necessary paths so spirv-builder can find cargo.

[LegNeato]

Great, thank you! 🚀

[tombh]

Great find @bspeice! So does this PR fix the issue you mentioned in your comment?

My only thought about this PR was whether each of the other platform ENV vars were also lists, and it looks like they are.

DYLD_FALLBACK_LIBRARY_PATH
This is a colon separated list of directories that contain libraries. It is used as the default location for libraries not found in their install path. By default, it is set to $(HOME)/lib:/usr/local/lib:/lib:/usr/lib.

LD_LIBRARY_PATH
List of directories where the system searches for runtime libraries in addition to those hard-defined by ld and in /etc/ld.so.conf.d/*.conf files.

So I approve this PR too ✅

[bspeice]

@tombh - yes, this resolves the "spirv-builder can't find cargo" issue in my comment.